Directional solidification is a method of casting using controlled cooling to cause a solidification interface to move progressively through a mold filled with molten metal. In the highest technology embodiment of this process, single crystal articles are cast. There are several ways in which single crystal castings in useful shapes can be formed. Common to all of them is that a solidification front having the desired crystal structure must be initiated within the molten metal of the casting. Sometimes very small and confined points of the mold are used, such as shown in U.S. Pat. No. 1,793,672 to Bridgman and Pat. No. 4,015,657 to Petrov et al. When a multiplicity of grains grow as adjacent columns, there is a progressive crowding out of certain grains with movement of the solidification front along the length of the casting. Thus, if the mold diameter is small enough and length great enough, eventually only a few grains, or a single grain, will remain. In U.S. Pat. Nos. 3,494,709 and 3,536,121 to Piearcey, an improved method is shown wherein a zig-zag passage in a mold causes the rapid selection of a single grain from a multiplicity of columnar grains. In a more refined practice, a helix shape is used, as shown in U.S. Pat. No. 4,111,252 to Day et al., it is this which has been widely used in commerce. However, in the methods referred to above there is no way to control the secondary orientation (orientation of the crystal within the x-y plane when growth is along the z axis). Additionally, the only grain orientation which will lie along the z axis direction of growth is that which nature preferentially selects, e.g., &lt;001&gt; in the case of a face centered cubic nickel superalloy.
Therefore, seed crystals are often used to initiate solidification fronts, to control crystal orientation and simplify mold construction. Typically, a seed crystal is placed at the base of the mold where it can be cooled. Molten metal poured into the mold contacts the seed, partially melting it. Thereafter, epitaxial solidification from the seed is caused to take place. The use of seed crystals is well known, as is shown by U.S. Pat. Nos. 3,763,926 to Tschinkel et al. and 3,857,436 to Petrov et al. In many instances solidification from seed crystals is preferred, and the present application is concerned with castings which are so made. Generally, the use of seeds has meant that there is no need for a zig zag or helix section. In some circumstances a narrowing of the mold above the seed crystal has been required. For example, in the co-pending application of U.S. Ser. No. 250,521 of Terklesen, filed Apr. 2, 1981, a specially shaped mold eases the difficulty of melting the seed crystal sufficiently to enable epitaxial solidification. Surplus molten metal is flowed across the seed crystal and thereafter surrounds it. Thus, there will be extraneous nucleation around the periphery of the seed crystal; it is Terklesen's practice to place a restriction in the mold above the seed crystal. In essence, the cross sectional area of the propagating solidification front is reduced to an area which is smaller than the cross section of the seed placed directly below the restriction. This stops vertical propagation into the article cavity of non-epitaxial crystal growth.
The foregoing techniques are effective in obtaining single crystal castings which are useful in commerce. As used herein, the term "single crystal casting" is a term of art and is not a rigid scientific term. Single crystal castings are functionally characterized by measuring the properties of the castings, and they may not have perfectly uniform crystal structure. Variations in crystallographic orientation are permissable when they do not substantially degrade mechanical properties below those of an ideally uniform crystal casting. See the co-pending U.S. Ser. No. 333,600 of Giamei, filed Dec. 23, 1981, the disclosure of which is hereby incorporated by reference. In the Giamei invention, a single crystal casting is formed by simultaneous solidification from two spaced apart seeds. As Giamei discloses, even in castings which are made from a single seed, it will be found that there is a progressive degradation in the crystallographic quality as a solidification front moves along the casting. Accordingly, a point in a casting which is distant from the seed crystal will be found to have more variation in crystal structure than will a location close to the seed. Points at the periphery of the casting at the distant location can significantly diverge in orientation from the nominal orientation of the casting. This phenomenon and its significance has only recently been appreciated, after continuing research and development.
Normally, divergence in crystal structure does not present a problem except in castings with large lateral dimensions. And such problems can be overcome by the aforementioned Giamei invention. But it is still desired to make more nearly ideal castings in general.
There is a further problem which involves the making of seeds. In some instances, it is possible to remove the original seed crystal from a just-made casting, and reuse it for another casting. But, more often the seed is difficult to retrieve for mechanical reasons, or it is unacceptably melted and merged with the casting. Thus seeds have been more effectively made as independent castings, followed by machining. But such a procedure is costly and improved techniques are desired.